1. Field of the Invention
The present invention is related to the field of digital image processing, and more specifically to filtering to remove ringing artifacts from decompressed images.
2. Description of the Related Art
Images are stored in arrays of pixels. Each pixel stores a small portion of an image, in the form of image data. The same is true for both static images (such as photographs), and for video sequences.
The large volume of image data slows down its transmission. To alleviate the problem, compression techniques are used to reduce the amount of data. Sometimes high compression is used for fastest transmission.
A problem arises when a high compression technique is used with insufficient bit rate of transmission. When the image is reconstructed, it includes visual noise, which is also called ringing artifacts. These artifacts are introduced in the places of the image where high spatial frequency DCT (Direct Cosine transform) components are substantially reduced. This occurs also around the dominant edges of the image.
The prior art has addressed this problem by applying a de-ringing filter, to remove the artifacts. This approach does not work satisfactorily. Either the artifacts are not removed fully, or the dominant edges are filtered too much. In that case, the dominant edges become so smoothened that they no longer appear as dominant, which detracts from the overall image quality.
The present invention overcomes these problems and limitations of the prior art.
Generally, the present invention provides a method for removing ringing artifacts from locations near dominant edges of a reconstructed image. The invention exploits the fact that artifacts near dominant edges has a direction that can be inferred from a main direction of the dominant edge. The invention teaches to filter the image data on either side of the pixels containing the edge. Filtering is with a special directional de-ringing filter according to the invention, which means that the de-ringing filter has a main direction aligned with the edge direction.
It will be appreciated that the dominant edges of the image are therefore not smoothened, since they themselves are not filtered. In addition, the artifacts are eliminated through a single filtering pass, since the directional de-ringing filter of the invention is fundamentally aligned with their main direction.
The invention will become more readily apparent from the following Detailed Description, which proceeds with reference to the drawings, in which: